1. Field
The present invention is generally directed to the therapeutic intervention of metabolic disorders, particularly those involving amino acid metabolism. More particularly, the present invention is directed to methods and compositions for the treatment of phenylketonuria, vascular diseases, ischemic or inflammatory diseases, or insulin resistance, or conditions and patients that would benefit from enhancement of nitric oxide synthase activity.
2. Background of the Related Technology
Phenylketonuria (PKU) is an inherited metabolic disorder that was first identified in the 1930s. In most cases, and until the mid-1990s, it was thought that this is a disorder of amino acid metabolism resulting from a deficiency in the liver enzyme phenylalanine hydroxylase (PAH). Deficiencies in PAH in turn result in an excess of phenylalanine (Phe) in the brain and plasma. The deficiency in PAH ultimately manifests in a lack of tyrosine, which is a precursor for the neurotransmitters.
Left undetected and untreated early in the life of an infant, PKU leads to irreversible damage of the nervous system, severe mental retardation and poor brain development. Features other than mental retardation in untreated patients include brain calcification, light pigmentation, peculiarities of gait, stance, and sitting posture, eczema, and epilepsy. It has been reported that an infant suffers a loss of 50 IQ points within the first year of infancy and PKU is invariably accompanied by at least some loss of IQ. Once detected, the condition is treated by providing the infant, and later the child, with a low Phe diet. In adults, the protein supplements routinely taken by classic PKU patients may be Phe-free with the assumption that such adults will receive sufficient quantities of Phe through the remaining diet, controlled under a strict regimen, so that the overall diet is a low Phe diet. Also, pregnant women who suffer from the condition are recommended a diet that is low in Phe to avoid the risk of impairment of the development of the fetus and congenital malformation (maternal PKU syndrome).
In more recent years it has been shown that pathological symptoms which manifest from the condition of excess of Phe, collectively termed hyperphenylalaninemia (HPA), may be divided into multiple discrete disorders, which are diagnosed according to plasma Phe concentrations and responsiveness to a co-factor for PAH. At an initial level, HPAs may be divided into HPA caused as a result of a deficiency in the cofactor 6R-L-erythro-5,6,7,8, tetrahydrobiopterin (BH4; malignant PKU) and HPA resulting from a deficiency in PAH. The latter category is further subdivided into at least three categories depending on the plasma concentration of Phe in the absence of dietary or other therapeutic intervention (referred to herein as “unrestricted plasma Phe concentration”).
Normal plasma Phe homeostasis is tightly controlled resulting in a plasma Phe concentration of 60 μmol/L±15 μmol/L. Classical PKU (OMIM No. 261600) is the most severe form of PKU and it results from null or severe mutations in PAH, which lead to unrestricted plasma Phe concentrations greater than 1200 μmol/L when left untreated. Individuals with classical (or severe) PKU must be treated with a strict dietary regimen that is based on a very low Phe diet in order to reduce their Phe concentrations to a safe range. Milder forms of HPA also have been characterized. A less severe form of PKU is one which manifests in plasma Phe concentrations of 10-20 mg/dL (600-1200 μmol/L), and is generally termed “mild PKU”. This moderate form of PKU is managed through the use of moderate dietary restrictions, e.g., a low total protein diet, but otherwise not necessarily Phe-free. Finally, mild HPA, also referred to as benign or non-PKU HPA is characterized by plasma Phe concentrations of between 180-600 μmol/L. The individuals with non-PKU HPA are not routinely treated as they are considered to have plasma Phe levels that are within the “safe” range. Nevertheless, as mentioned above, these Phe levels are still significantly elevated in these individuals as compared to normal, non-PKU subjects and may present detrimental sequelae in at least pregnant women and very young patients. For a more detailed review of HPA resulting from PAH deficiency, those of skill in the art are referred to Scriver et al., 2001 (Hyperphenylalaninemia: Phenylalanine Hydroxylase Deficiency, In: Scriver C R, Beaudet A L, Sly W S, Valle D, Childs B, Vogelstein B, eds. The Metabolic and Molecular Bases of Inherited Disease. 8th ed. New York: McGraw-Hill, 2001: 1667-1724). NIH Guidelines indicate that for children with PKU, it is preferable reduce the plasma Phe to be 360-420 μmol/L.
HPA also results from defects in BH4 metabolism. BH4 is an essential cofactor of both tyrosine and tryptophan hydroxylase, the rate limiting enzymes in the biosynthesis of the neurotransmitters dopamine and serotonin. The effects of deficiencies in dopamine and serotonin are collectively known as “atypical” or “malignant” HPA. Thus, traditional diagnoses of HPA have involved a determination of whether the HPA is a result of BH4 deficiency or PAH deficiency. Typically, diagnosis of PKU is established on the basis of a persistently elevated blood Phe concentration. Following a positive screen for elevated blood Phe (plasma Phe>120 μmol/L; Weglage et al., J. Inherit. Metab. Dis., 25:321-322, 2002), a differential screen is performed in which it is determined whether the elevated Phe is a result of BH4 deficiency or PAH deficiency. The differential diagnosis involves determining whether the elevated Phe concentration is decreased as a result of BH4 administration (BH4 loading test). The BH4 loading test typically involves a one-time load of BH4 e.g., 5-20 mg/kg being administered to the subject who is on a normal (i.e., unrestricted) diet and determining whether the subject experiences a decrease in Phe levels (see e.g., Ponzone et al., Eur. J. Pediatr. 152:655-661, 1993; Weglage et al., J. Inherit. Metab. Dis., 25:321-322, 2002.)
Typically, individuals that respond to a BH4 loading test by a decrease in plasma Phe levels are diagnosed as having a defect in BH4 homeostasis. However, there have been various reports of patients with a BH4 responsive type of PAH deficiency (Kure et al., J. Pediatr. 135:375-378, 1999; Lassker et al., J. Inherit. Metabol. Dis. 25:65-70, 2002; Linder et al., Mol. Genet. Metab. 73:104-106, 2001; Spaapen et al., Mol. Genet. and Metabolism, 78:93-99, 2003; Trefz et al., 2001). These subjects have plasma Phe levels that are typical of moderate PKU, i.e., less than 1000 μmol/L and typically less than 600 μmol/L. Patients that have severe classical PKU are not responsive to typical 24 hour BH4 loading tests (Ponzone et al., N. Engl. J. Med 348(17):1722-1723, 2003).
It has been suggested that individuals that are responsive to BH4 do not require dietary intervention, but rather should be treated with BH4. Likewise, the converse has been suggested for subjects that have been diagnosed as non-responsive to the BH4 loading test, i.e., these subjects should be treated with dietary restriction and not BH4 therapy. Ponzone et al. particularly noted that individuals that have severe phenylketonuria will not respond to BH4 therapy and such therapy should not be used on these patients (Ponzone et al., N. Engl. J. Med 348(17):1722-1723, 2003). Thus, presently there are divergent therapeutic regimens for treatment of HPA depending on whether or not the individual is responsive to BH4. Moreover, it has been suggested that very few patients will benefit from BH4 therapy. In fact, it is thought that the only individuals with a PAH-deficient form of HPA that will benefit from BH4 therapy are those with mild PKU. As these individuals will typically have Phe levels in the safe range (i.e., less than 600 μM), the disease state can be controlled using moderate dietary restriction (see Hanley, N. Engl. J. Med 348(17):1723, 2003). Thus, BH4 therapy either alone, or in combination with any other therapeutic intervention, has not being considered as a viable therapeutic intervention for the vast majority of individuals with HPA.
BH4 is a biogenic amine of the naturally-occurring pterin family. Pterins are present in physiological fluids and tissues in reduced and oxidized forms, however, only the 5,6,7,8, tetrahydrobiopterin is biologically active. This is a chiral molecule and the 6R enantiomer of the cofactor is known to be the biologically active enantiomer. For a detailed review of the synthesis and disorders of BH4 see Blau et al., 2001 (Disorders of tetrahydrobiopterin and related biogenic amines. In: Scriver C R, Beaudet A L, Sly W S, Valle D, Childs B, Vogelstein B, eds. The Metabolic and Molecular Bases of Inherited Disease. 8th ed. New York: McGraw-Hill, 2001: 1275-1776). Despite the elucidation of the role of BH4 deficiency in HPA, treatment with BH4 has not been suggested because such treatment is very expensive, as high as $30,000 per year for an adolescent or adult, as compared with $6,000 for phenylalanine-restricted dietary therapy (Hanley, N. Engl. J. Med 348(17):1723, 2003). Another significant problem with BH4 is that this compound is unstable and readily undergoes aerobic oxidation at room temperature (Davis et al., Eur. J. Biochem., Vol 173, 345-351, 1988; U.S. Pat. No. 4,701,455) and has a shelf-life of less 8 hours at room temperature (Berneggar and Blau, Mol. Genet. Metabol. 77:304-313, 2002).
Thus, to date, dietary intervention is the typical therapeutic intervention used for all patients with severe classical PKU and in many patients with moderate PKU. Such dietary intervention typically entails restricting the patient to foodstuff that is composed of natural foods which are free from, or low in, Phe. However, in addition to eliminating Phe, such a dietary regimen eliminates many sources of other essential amino acids, vitamins and minerals. Consequently, without supplementation, such a diet provides inadequate protein, energy, vitamins and minerals to support normal growth and development. As PKU is a manifestation of a lack of tyrosine, which arises due to the lack of hydroxylation of phenylalanine, tyrosine becomes an essential amino acid and dietary supplements for PKU must contain a tyrosine supplement. Therefore, it is common to use nutritional formulas to supplement the diets of PKU patients. Also, for babies, it is common to use infant formulas which have a low Phe content as the sole or primary food source.
However, dietary protein restriction is at best an ineffective way of controlling PKU in many classes of patients. For example, treatment is of paramount importance during pregnancy because high Phe levels may result in intrauterine retardation of brain development. However, a low protein diet during pregnancy may result in retarded renal development and is thought to produce a subsequent reduction in the number of nephrons and potentially leads to hypertension in adulthood. (D'Agostino, N. Engl. J. Med. 348(17)1723-1724, 2003).
Poor patient compliance with a protein-restricted diet also is a problem. The Phe-free protein formulae available are bitter tasting making it difficult to ensure that the patient consumes sufficient quantities of the protein to maintain the required daily intakes of protein, amino acids, vitamins, minerals, and the like. This is particularly a problem with older children who may require up to 70 g, dry weight, of the formulas per day. For example, Schuett, V. E.; 1990; DHHS Publication No HRS-MCH-89-5, reports that more than 40% of PKU patients in the US of eight years or older no longer adhere to the dietary treatment. (U.S. Pat. No. 6,506,422). Many adolescent patients fail to rigorously follow the protein-restricted diet due to fears of peer attitude.
Thus, there remains a need for a therapeutic medicament to replace or supplement and alleviate the dietary restrictions under which a PKU patient is placed. The present invention is directed to addressing such a need.